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Sunday, April 1, 2007

Digital Exposure Essentials

Getting the exposure right is at least as important when shooting digital as when shooting film

Digital Exposure Essentials

Exposure Lock

One way to deal with a complex lighting situation is to use exposure lock. If the scene you’re trying to photograph has a wide range of light and dark objects, for example, metering something that’s midtone (or close to middle gray) often will be a much better interpretation of the natural light. Sometimes metering the sky is a good option, as long as you’re not facing the sun. It’s always wise to experiment. Try a number of different places to point your camera and lock the exposure before you reframe and take each shot.


The single best tool you have at your disposal for determining the right combination of ƒ-stop and shutter speed is the histogram. Not all digital cameras have this feature, but if yours does, you definitely should use it because a histogram shows the exact range of tones captured by the image sensor. A photo’s exposure can look good on the LCD monitor, but upon later review on your computer, it actually can be quite poor. The goal always should be to get the best possible photo in the first place, and the histogram makes this easy.

Represented as a graph, a histogram looks like a mountain range. The left side depicts the darkest parts of the photo and the right side depicts the lightest. Anything beyond the left edge is pure black and anything beyond the right edge is pure white—both are outside the range of the image sensor.

Every histogram will be different and there’s no "correct" shape. Depending on which is more prevalent in your photograph, shadows or highlights, the histogram visually may favor one side or the other. Ideally, the slope of dark or light tones should end at or before the edge of the graph. What you have to watch for is the slope of the graph being abruptly cut off at either edge. When this happens, this means details aren’t being captured by the sensor.

To make a quick adjustment if you see a problem, you can use the ± exposure compensation control. This feature allows you to manually adjust the brightness of the auto-exposure value set by the camera before you take the next shot.

If you’re shooting in one of your camera's manual modes, work from the camera’s recommended settings for ƒ-stop and/or shutter speed, then vary them from there based on what the images look like on the histogram. Refine your settings and experiment. There’s certainly more than one way to get a proper exposure, with each having a different tonal range. The same scene could be captured with any number of ƒ-stop and shutter-speed combinations.

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